Some properties of dust outside the galactic disk

Abstract

Pulkovo Astronomical Observatory, Russian Academy of Sciences, Pulkovskoe sh. 65, St. Petersburg, 196140 Russia Key words: interstellar dust grains; color-magnitude diagrams, giant and subgiant starsThe joint use of accurate near-and mid-infrared photometry from the 2MASS and WISE catalogues has allowed the variations of the extinction law and the dust grain size distribution in high Galactic latitudes (|b| > 50• ) at distances up to 3 kpc from the Galactic midplane to be analyzed. The modified method of extrapolation of the extinction law applied to clump giants has turned out to be efficient for separating the spatial variations of the sample composition, metallicity, reddening, and properties of the medium. The detected spatial variations of the coefficients E (H−W 1) /E (H−Ks) , E (H−W 2) /E (H−Ks) , and E (H−W 3) /E (H−Ks) are similar for all high latitudes and depend only on the distance from the Galactic midplane. The ratio of short-wavelength extinction to long-wavelength one everywhere outside the Galactic disk has been found to be smaller than that in the disk and, accordingly, the mean dust grain size is larger, while the grain size distribution in the range 0.5 − 11 microns is shifted toward coarse dust. Specifically, the mean grain size initially increases sharply with distance from the Galactic midplane, then decreases gradually, approaching a value typical of the disk at |Z| ≈ 2.4 kpc, and, further out, stabilizes or may increase again. The coefficients under consideration change with coordinate Z with a period of about 1312 ± 40 pc, coinciding every 656 ± 20 pc to the south and the north and showing a significant anticorrelation between their values * E-mail: georgegontcharov@yahoo.com 1 in the southern and northern hemispheres at intermediate Z. Thus, there exists a unified large-scale periodic structure of the interstellar medium at high latitudes within at least 5 kpc. The same periodic variations have also been found for the extinction coefficient R V within 600 pc of the Galactic midplane through the reduction of different photometric data for stars of different classes. 2 INTRODUCTIONThe wavelength dependence of interstellar extinction (extinction law) is related primarily to the dust grain size distribution in the absorbing medium. In the most widespread media, the extinction maximum in a region of space occurs at a wavelength that coincides in order of magnitude with the mean dust grain diameter in this region (Bochkarev 2010). For example, the commonly considered extinction A V in the photometric V band with a wavelength of 0.55 microns and, in general, the visual extinction is produced predominantly by submicron sized dust grains, whereas the infrared (IR) one is produced by supermicron ones. Conclusions about the physical properties of dust in the region of space under consideration and about the contribution of dust to the total mass of the matter can be reached by studying the wavelength dependence of extinction. Conversely, the extinction can be calculated by studying the properti...

Interstellar Extinction

“…Only one their result applies to the space far from the galactic plane: in the direction of three high-latitude clouds at distances of 0.5 − 1 kpc from the galactic plane they found "grey" extinction by 0.2 m − 0.4 m using SDSS data, a value in agreement with the results of Gontcharov (2013bGontcharov ( , 2016.…”

“…At latitudes 50 • < b < 90 • , the relative extinction in the H and K s bands decreases, but the extinction increases strongly in W 1 and W 2. The same has been shown by Gontcharov (2013bGontcharov ( , 2016. This shows up as a gradual increase in the average size of the dust particles with distance from the galactic plane.…”

“…8 that the extinction in the galactic halo found by Gontcharov (2013bGontcharov ( , 2016 in the W 1 and W 2 bands exceeds the other results mentioned here by more than the declared errors. However the results of Gontcharov (2013bGontcharov ( , 2016 were obtained over a much greater distance from the galactic plane, for which there are almost no other studies for comparison.…”

“…With the same zero point, the results of Gontcharov (2013bGontcharov ( , 2016 It can be seen in Fig. 8 that the extinction in the galactic halo found by Gontcharov (2013bGontcharov ( , 2016 in the W 1 and W 2 bands exceeds the other results mentioned here by more than the declared errors.…”

“…This can be seen in 2015), have found the same behavior in the galaxy M82 (R V initially decreases with distance from the galactic center and then increases for many kpc). Gontcharov (2012aGontcharov ( , 2013bGontcharov ( , 2016 has analyzed the extinction law in spatial cones of height 25 kpc extending from the Sun to the galactic poles. This made use of photometric data on 9769 clump giants (in a radius of 8 • around the poles) and 1221 branch giants (in a radius of 20 • around the poles) from the Tycho-2, 2MASS, and WISE catalogs.…”